Method of reducing an amount of mercury in a flue gas

ABSTRACT

The subject matter disclosed herein relates to a method of reducing an amount of mercury discharged to an environment in a flue gas ( 12 ) generated by combustion of a fuel source. The method includes contacting the flue gas with a moist pulverous material upstream of a particle separator ( 24 ), mixing powdered activated carbon (PAC) in an amount between about 0.5 lb/MMacf and 10 lbs/MMacf with the flue gas ( 12 ) upstream of the particle separator ( 24 ), wherein the PAC interacts with at least a portion of mercury containing compounds in the flue gas ( 12 ), and separating the mercury containing compounds from the flue gas ( 12 ) containing the moist pulverous material and PAC, thereby reducing an amount of mercury in the flue gas ( 12 ).

BACKGROUND

(1) Field

The disclosed subject matter generally relates to reducing an amount ofmercury discharged to an environment incident to the combustion of afuel source containing mercury or mercury containing compounds, and moreparticularly to reducing an amount of mercury discharged in a combustionflue gas that is subjected to a flash dryer absorber (FDA) system.

(2) Description of the Related Art

Combustion of fuel sources such as coal produces a waste gas, referredto as “flue gas” that is to be emitted into an environment, such as theatmosphere. The fuel sources typically contain sulfur and sulfurcompounds, which are converted in the combustion process to gaseousspecies, including sulfur oxides, which then exist as such (otherwiseknown as “acid gases”) in the resulting flue gas. The fuel sourcestypically also contain elemental mercury or mercury compounds, which areconverted in the combustion process to, and exist in the flue gas as,gaseous elemental mercury or gaseous ionic mercury species (generallyreferred to hereinafter as “mercury containing compounds”).

Accordingly, flue gas contains dust, fly ash, and noxious substancessuch as acid gases, as well as other impurities, that are considered tobe environmental contaminants. Prior to being emitted into theatmosphere via a smoke stack (“stack”), the flue gas undergoes acleansing or purification process.

In coal combustion, flue gas often undergoes a desulfurization process,which typically occurs in a flue gas desulfurization system. There areseveral types of desulfurization systems, including wet flue gasdesulfurization (WFGD), also known as “wet scrubbers” and dry flue gasdesulfurization (DFGD), also known as “dry scrubbers.” There are twoseparate types of DFGD, the first is a spray dryer absorber (SDA), whilethe other is a flash dryer absorber (FDA).

Acid gases are removed from flue gas using a FDA system by chemicallyreacting a moist pulverous material with the acid gases contained withinthe flue gas. Generally, the acid gases are absorbed by the moistpulverous material, which is then separated from the flue gas by theparticle separator. The moist pulverous material typically includes0.5-5 wt. % water based on the total weight of the moist pulverousmaterial and a basic reagent that will interact with contaminants toremove them from the flue gas. Examples of basic reagents that areuseful in the moist pulverous material include, but are not limited to,particulate material collected from the flue gas (such as dust and flyash), as well as alkaline material, which generally can be selected fromlime, limestone, calcium hydroxide and the like and combinationsthereof.

Recently, there has been an increased focus on the removal of mercury.Presently, there are various methods for removing mercury from flue gasemissions. Those methods include, but are not limited to the following:addition of oxidizing agents in a boiler upstream of the flue gasemission control system and then removing it with wet scrubbers;addition of reactants to bind mercury and remove it from the flue gas;and utilization of particular coal or fuel that minimizes the amount ofmercury released when the coal or fuel is burned.

It has been shown that a number of generally known methods of mercuryremoval are effective to produce mercury salts, which can be dissolvedand removed by the aqueous alkaline slurry used in a wet flue gasdesulfurization system (WFGD). Some of these methods include theaddition of halogen or halogen compounds, such as bromine, to the coalor to the flue gas upstream of the wet scrubbing operation, to provideoxidation of elemental mercury to ionic mercury and formation of mercurysalts, which are then dissolved in the aqueous alkaline slurry incidentto the sulfur oxide removal processes. However, the removal of mercuryin a DFGD system has proven to be difficult to control and it is noteasily predicted when designing a flue gas cleaning system with respectto mercury removal. The desired emission guarantee levels are often aslow as 1 μg/Nm³ of mercury, which corresponds to a very efficientmercury removal in the DFGD system.

SUMMARY

One aspect of the disclosed subject matter relates to a method ofreducing an amount of mercury discharged to an environment in a flue gasgenerated by combustion of a fuel source. The method includes:contacting the flue gas with a moist pulverous material upstream of aparticle separator, wherein the moist pulverous material facilitates theremoval of acid gases from the flue gas; mixing powdered activatedcarbon (PAC) in an amount between about 0.5 lb/MMacf and 10 lbs/MMacfwith the flue gas upstream of the particle separator, wherein the PACinteracts with at least a portion of mercury containing compounds in theflue gas; and separating the mercury containing compounds from the fluegas containing the moist pulverous material and PAC, thereby reducing anamount of mercury in the flue gas.

Another aspect of the disclosed subject matter relates to a system forreducing an amount of mercury discharged to an environment in a flue gasgenerated by combustion of a fuel source. The system includes: a mixeradapted to form a moist pulverous material effective to removecontaminants from the flue gas, wherein the mixer facilitates theintroduction of the moist pulverous material to the flue gas; a particleseparator downstream of the mixer; and means for introducing powderedactivated carbon (PAC) to the flue gas upstream of the particleseparator in an amount between about 0.5 lbs/MMacf and 10 lbs/MMacf toremove at least a portion of mercury containing compounds from the fluegas, thereby reducing an amount of mercury discharged to an environment.

Another aspect of the disclosed subject matter relates to a method ofremoving mercury from a flue gas generated by combustion of a fuelsource. The method includes: forming a moist pulverous materialcomprising at least one alkaline material selected from lime, limestone,calcium hydroxide and combinations thereof; introducing the moistpulverous material to the flue gas upstream of a particle separator;introducing powdered activated carbon (PAC) to the flue gas tofacilitate the removal of at least a portion of mercury containingcompounds, wherein the PAC is mixed with the flue gas upstream of theparticle separator in an amount between 0.5 lbs/MMacf and 10 lbs/MMacf;and separating at least a portion of the mercury containing compoundsfrom the flue gas in the particle separator, thereby removing mercuryfrom the flue gas.

Yet another aspect of the disclosed subject matter relates to a methodof reducing an amount of mercury discharged to an environment in a fluegas generated by combustion of a fuel source. The method includes:contacting the flue gas with a moist pulverous material, wherein themoist pulverous material facilitates the removal of acid gases from theflue gas; mixing powdered activated carbon (PAC) in an amount betweenabout 0.5 lb/MMacf and 10 lbs/MMacf with the flue gas, wherein the PACinteracts with at least a portion of mercury containing compounds in theflue gas; and separating the mercury containing compounds from the fluegas containing the moist pulverous material and PAC, thereby reducing anamount of mercury in the flue gas.

Another aspect of the disclosed subject matter relates to a system forreducing an amount of mercury discharged to an environment in a flue gasgenerated by combustion of a fuel source. The system includes: a contactmodule for contacting flue gas with a moist pulverous material, whereinsaid moist pulverous material facilitates the removal of acid gases fromsaid flue gas; a PAC module for mixing powdered activated carbon (PAC)in an amount between about 0.5 lb/MMacf and 10 lbs/MMacf with said fluegas, wherein said PAC interacts with at least a portion of mercurycontaining compounds in said flue gas; and a separation module forseparating said mercury containing compounds from said flue gascontaining said moist pulverous material and PAC, thereby reducing anamount of mercury in said flue gas.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the subject matter disclosed herein, thedrawing shows a form of the embodiments that is presently preferred.However, it should be understood that the disclosed subject matter isnot limited to the precise arrangements and instrumentalities shown inthe drawing, wherein:

FIG. 1 is a schematic diagram of a system for reducing an amount ofmercury emitted by a flue gas, which is practiced using a DFGD system;

FIG. 2. is a schematic diagram of a system for reducing an amount ofacid gases emitted by a flue gas, which is practiced using a DFGD systemas known in the prior art;

FIG. 3 illustrates a process stream of a flue gas generated bycombustion of a fuel source as known in the prior art;

FIG. 4 illustrates a process stream of a flue gas generated bycombustion of a fuel source; and

FIG. 5 illustrates a process stream of a flue gas generated bycombustion of a fuel source.

DETAILED DESCRIPTION

Referring now to the drawings in which like reference numerals indicatelike parts, and in particular to FIG. 1, one aspect of the disclosedsubject matter is a system 10 for reducing an amount of gaseouselemental mercury emitted by a flue gas, which is practiced using a dryscrubbing operation. The components within system 10, which aredescribed in more detail herein, are typically controlled by acontroller 11. Controller 11 is configured in a manner to allow theaddition of materials and the fluid communication between components ofsystem 10.

In system 10, a flue gas 12 travels from a combustion source, such as acoal-fired boiler 20, through a duct 22 to various equipment designed toremove contaminants from the flue gas. In duct 22, flue gas 12 istypically at a temperature between about 120° C. and about 200° C.

In addition to particulate material such as dust and fly ash, as well asgaseous contaminants such as sulfur oxide, upon leaving boiler 20 fluegas 12 may contain mercury containing compounds and have a mercuryconcentration of about 5 μg/Nm3 to about 200 μg/Nm³.

After traveling from boiler 20, flue gas 12 enters a particle separator24, which separates and removes particulate material, includingparticulate material having contaminants, such as acid gas, absorbedthereon, from the flue gas. The term “particulate material” as usedherein, includes, but is not limited to, dust, fly ash, as well asmaterial having contaminants absorbed thereon. Particle separator 24also facilitates the re-circulation of at least a portion of theparticulate material into system 10.

In FIG. 1, particle separator 24 is shown as a fabric filter, whichincludes a plurality of rows of filter bags through which the flue gaspasses and is cleaned. It is contemplated that other types of particleseparators, such as an electrostatic precipitator, can be used in thepresently described system.

The cleaned flue gas, i.e., flue gas 12′, eventually exits particleseparator 24 and is sent through a duct 26 to stack 28, where it isreleased to the atmosphere. Upon exiting particle separator 24, flue gas12′ typically is at a temperature between about 60° C. and about 90° C.and contains less than 0.03 g/Nm³ of particles and has an acceptableconcentration of sulfur oxides.

After passing through particle separator 24 and prior to its emissioninto the atmosphere, flue gas 12′ may be subjected to other treatmentprocesses or equipment to remove contaminants therefrom.

Duct 22 includes a vertical portion 30, through which flue gas 12 flowsto reach particle separator 24. A mixer 32 is in fluid communicationwith vertical portion 30, which serves as an area or module where fluegas 12 can come into contact and react with contaminant-removingreagents that are prepared in the mixer. Mixer 32 can be any apparatusthat facilitates the mixture of reagents to form a moist pulverousmaterial and the introduction of the same into flue gas 12. One exampleof mixer 32 is the Alstom FDA system, manufactured by Alstom Power,Knoxville, Tenn.

Mixer 32 typically has a chamber to which various reagents and water areadded. Mixer 32 may have a mechanical mixing mechanism (not shown)having agitators for combining the reagents and water.

Mixer 32 introduces the moist pulverous material into flue gas 12 invertical portion 30. The moist pulverous material can be introduced toflue gas 12 by any means known in the art, including an injectionmechanism, a chute, a valve, and the like. In addition to facilitatingthe removal of contaminants from flue gas 12, the moist pulverousmaterial lowers the temperature of the flue gas to a temperatureconducive to facilitate such a removal.

The moist pulverous material can be introduced to flue gas 12 in acontinuous manner or can be introduced in an amount sufficient to absorband remove contaminants therefrom. The particular amount of moistpulverous material added to flue gas 12 is determined by variables ineach system, and such determination can be readily made by a systemoperator.

In some embodiments, as shown in FIG. 1, the moist pulverous materialincludes water and particulate material, and is formed by mixing waterfrom a supply line 34 with a portion of particulate material (not shown)separated from flue gas 12 by particle separator 24 and transported fromthe particle separator to mixer 32 via a duct 36. The remainingparticulate material from particle separator 24 not sent to mixer 32 canbe used in various manners or discarded. The moist pulverous materialtypically contains about 0.5-5 wt. % water based on the total weight ofthe moist pulverous material. However, in one example the moistpulverous material contains about 1-2 wt. % water based on the totalweight of the moist pulverous material.

In another embodiment, the moist pulverous material additionallycontains an alkaline material 38. Alkaline material 38 can be added tothe particulate material separated from flue gas 12 by particleseparator 24 in mixer 32 and combined with water. Alkaline material 38can be any alkaline material, such as lime, limestone, calcium hydroxideand the like, and combinations thereof. Alkaline material 38 can bestored in a separate tank 40 and introduced to mixer 32 on a continuousor as-needed basis. The moist pulverous material having alkalinematerial 38 mixed therein typically contains 0.5-5 wt. % water based onthe total weight of the moist pulverous material. In one example, themoist pulverous material contains about 1-2 wt. % water based on thetotal weight of the moist pulverous material.

In some systems, alkaline material 38 can be introduced to system 10independently of the moist pulverous material. For example, alkalinematerial 38 can be directly introduced to boiler 20. In such anembodiment, alkaline material 38 is present in flue gas 12 when themoist pulverous material is introduced to the flue gas. In such asystem, alkaline material 38 is not introduced into mixer 32.

The moist pulverous material is typically introduced to flue gas 12 viamixer 32 and subsequently facilitates the removal of contaminants fromthe flue gas. Contaminants that are removed from flue gas 12 include,but are not limited to, sulfur oxides. As shown in FIG. 2, to effectmercury removal from the flue gas, powdered activated carbon (PAC) isadded to system 10.

Referring now to FIG. 2, PAC can be added to flue gas 12 through avariety of modules. In one embodiment, PAC can be added to the moistpulverous material made in mixer 32. PAC is added to mixer 32 via supplyline 42. Any type of PAC, including PAC that is impregnated with ahalogen or sulfur, can be utilized in the instant system.

Upon introduction of the moist pulverous material and PAC into verticalportion 30, the mercury present in flue gas 12 will interact, and react,with PAC and be separated from the flue gas upon collection of the PACin particle separator 24. Typically, flue gas 12′ has a mercuryconcentration of about 1 μg/Nm³ or less.

Alternatively, PAC can be introduced into the flue gas at a pointupstream of mixer 32, which is illustrated as point 44 in FIG. 2. Inthis embodiment, PAC is introduced to the flue gas independently of themoist pulverous material formed in mixer 32.

Still referring to FIG. 2, in either embodiment, PAC is introduced tothe flue gas at an injection rate between about 0.5 and 10 lbs/MMacf.

Referring now to FIG. 3, some embodiments include a method 90. At 100,flue gas 12 is generated by a boiler 20. Flue gas 12 is sent throughsystem 10 via duct 22. Within system 10, as shown at 102, a moistpulverous material, as described above, is added to flue gas 12. Themoist pulverous material effects the removal of contaminants, such asacid gases, from flue gas 12.

After the addition of the moist pulverous material, at 104, flue gas 12travels to particulate separator 24, which removes particulates andother contaminants, from the flue gas thus forming a clean flue gas,flue gas 12′. Subsequently, at 106, flue gas 12′ is exhausted to theatmosphere via stack 28.

To effect the removal of mercury from flue gas 12, in some embodiments,PAC is added to the flue gas. Referring now to FIG. 4, some embodimentsinclude a method 190. After flue gas 12 is generated by combustion of afuel source at 200, the flue gas travels via duct 22 to a verticalportion 30, which is in fluid communication with mixer 32. Mixer 32facilitates the addition of the moist pulverous material and PAC to fluegas 12 at 202. Subsequently, and similarly to the process illustrated inFIG. 3, flue gas travels to particulate separator 24 at 204. Afterpassing through particulate 24, flue gas 12 is cleansed, and isexhausted to the atmosphere via stack 28 at 206.

Now referring to FIG. 5, which also illustrates the addition of PAC tothe flue gas, some embodiments include a method 290. At 300, flue gas 12is generated by combustion of a fuel source and travels through system10 by passing through duct 22. At 302, PAC is added to flue gas 12 whileit travels through duct 22. Subsequent to the addition of PAC, at 304,moist pulverous material is added to flue gas 12. Flue gas 12 travels toparticulate separator at 306, which removes particulates and othercontaminants therefrom, thereby forming clean flue gas 12′, which at308, is exhausted to an atmosphere via stack 28.

Although the subject matter has been described and illustrated withrespect to exemplary embodiments thereof, it should be understood bythose skilled in the art that the foregoing and various other changes,omissions and additions may be made therein and thereto, without partingfrom the spirit and scope of the disclosed method and system.Accordingly, other embodiments are within the scope of the followingclaims.

1. A system for reducing an amount of mercury discharged to anenvironment in a flue gas generated by combustion of a fuel source, saidsystem comprising: a mixer adapted to form a moist pulverous materialeffective to remove contaminants from said flue gas, wherein said mixerfacilitates the introduction of said moist pulverous material to saidflue gas; a particle separator downstream of said mixer; and means forintroducing powdered activated carbon (PAC) to said flue gas upstream ofsaid particle separator in an amount between about 0.5 lbs/MMacf and 10lbs/MMacf to remove at least a portion of mercury containing compoundsfrom said flue gas, thereby reducing an amount of mercury discharged toan environment.
 2. The system of claim 1, wherein said PAC comprises atleast one compound selected from a halogen, sulfur, and combinationsthereof.
 3. The system of claim 1, wherein said moist pulverous materialincludes: a mixture of: water; particulate material removed from saidflue gas by said particle separator; and at least one alkaline materialselected from lime, limestone, calcium hydroxide, and combinationsthereof.
 4. The system of claim 1, wherein said moist pulverous materialincludes: a mixture of: water; and particulate material removed fromsaid flue gas by said particle separator.
 5. The system of claim 1,wherein said PAC is introduced to said flue gas at a point upstream ofsaid mixer.
 6. The system of claim 1, wherein said PAC is introduced tosaid moist pulverous material in said mixer, wherein said PAC and saidmoist pulverous material are subsequently introduced to said flue gas.7. A system for reducing an amount of mercury discharged to anenvironment in a flue gas generated by combustion of a fuel source, saidsystem comprising: a contact module for contacting flue gas with a moistpulverous material, wherein said moist pulverous material facilitatesthe removal of acid gases from said flue gas; a PAC module for mixingpowdered activated carbon (PAC) in an amount between about 0.5 lb/MMacfand 10 lbs/MMacf with said flue gas, wherein said PAC interacts with atleast a portion of mercury containing compounds in said flue gas; and aseparation module for separating said mercury containing compounds fromsaid flue gas containing said moist pulverous material and PAC, therebyreducing an amount of mercury in said flue gas.